A conventional MEMS resonator will be described, with reference to FIG. 16 and FIG. 17. FIG. 16 is a perspective view illustrating a conventional MEMS resonator 100 which is formed using an SOT (Silicon On Insulator) substrate. A beam-type vibrator 101, an input electrode 102 and an output electrode 103 have been formed by etching an uppermost Si layer in the SOI. The vibrator 101 which is supported at its opposite ends is formed by etching a portion of a BOX (Buried Oxide) layer, in a state where the vibrator 101 is allowed to vibrate. Through the remainder portion of the BOX layer, supporting portions 104 at the opposite ends of the vibrator 101, the input electrode 102 and the output electrode 103 are anchored to a silicon substrate 105, with an embedded oxide layer 110 interposed therebetween.
FIG. 17 is a view schematically illustrating a cross section of the conventional MEMS resonator illustrated in FIG. 16, taken along the line A-A. As illustrated in FIG. 17, the conventional MEMS resonator 100 is structured such that the electrodes 102 and 103 are placed to face the respective side surfaces of the vibrator 101 in the opposite sides, with gaps interposed therebetween. One of the electrodes is made to be the input electrode 102, while the other electrode is made to be the output electrode 103. As a concrete example, by applying a DC voltage Vp to the vibrator 101, it is possible to realize a structure which provides a DC electric-potential difference between the input electrode 102 and the vibrator 101 and, also, provides a DC electric-potential difference between the output electrode 103 and the vibrator 101.
In the MEMS resonator 100, if an AC voltage is applied to the input electrode 102, this induces a change of the electric-potential difference between the input electrode 102 and the vibrator 101, which causes an excitation force derived by an electrostatic force to be applied to the vibrator 101. When the frequency of the AC voltage applied to the input electrode 102 is coincident with the mechanical resonance frequency of the vibrator 101, the vibrator 101 is largely vibrated, which causes the output electrode 103 to output an electric current due to the displacement along with the vibration thereof. This is the principle of operations of the MEMS resonator 100.
The vibrator 101 illustrated in FIG. 16 has a both-ends-supported beam shape which is fixed at its opposite ends and is held in a midair state at its middle portion. If an electrostatic force is applied to the vibrator 101, this excites a flexural vibration mode in which the beam induces flexural vibrations therein. As a method for fabricating an MEMS resonator employing an SOI as described above, there has been a common method described in “Non-patent Literature 1” which will be described later, for example. Further, “Patent Literature 1”, which will be described later, describes a vibrator having a disk shape. Resonance modes which can be excited in the vibrator include torsional vibration modes as described in “Patent Literature 2” which will be described later, in addition to flexural vibration modes, wherein such torsional vibration modes can be excited even when the vibrator has a beam shape.